A bank card is known, in particular from U.S. Pat. No. 5,627,355, which is incorporated herein by reference, that includes a switch actuatable by a sort of push-button or a deformable zone of the card, so that the exertion of pressure in the zone of the card containing the switch starts a specific secure identification algorithm. This card further includes at least one digital display for displaying in series various personal identification numbers, or PIN codes. This card also includes an electronic unit and an electric power source associated with a solar cell arranged on the surface of the card. This type of card can be used in various services, whether for making telephone calls from public telephones, a service for downloading films from the Internet, or accessing a bank or postal account, particularly so as to perform various transactions, such as the payment of bills.
The identification program provided in U.S. Pat. No. 5,627,355 consists in storing a list of PIN codes in the card, generated uniquely for a given client, the same list being stored in the bank's server. Each time the card is actuated by the switch, a new personal code is selected in the list and displayed. This selection generally consists in taking the next unused code from the pre-established list. The user has to enter the code in the machine or computer that transmits it to the server. The server checks that the code is actually the next unused code in the list for the client in question. This is known as a “one time password” or OTP. The importance of maintaining synchronisation between the position in the card list and the position in the server's list is thus clear. If codes in the card list are used without being transmitted to the server, particularly when the card is inadvertently actuated, the code transmitted during the next transaction with the server will not match the next unused code in the server's list. U.S. Pat. No. 5,627,355 mentions a method for partially overcoming this problem, via comparison with a code sequence in the server's list so as to detect any loss of synchronisation and thus re-establish the match between the two lists. This method can only be limited to a short sequence, particularly for security reasons. Consequently, if several codes are wasted in the card, the system will no longer accept the codes supplied by the client. Then, even if the match is regularly updated, usable codes are wasted and there is thus a smaller number of possible transactions for a given list stored in the card. The card will thus be used up more quickly. Finally, another drawback connected to this problem arises from the fact that inadvertent actuations of the card result in a wasteful increase in the consumption of the electric energy stored in the card.
In general, a first switch actuation of a card of the type described above is for initialising the application. Next, each new cycle is started by each new actuation of the switch. The problem connected to inadvertent actuations of the switch and thus of the secure identification program concerns not only the card user, who, for example, places the card in a wallet in his back trouser pocket and who, when sitting down, may sometimes unintentionally actuate the card switch, but also production steps during which pressure is exerted on the card, particularly when outer layers are added by laminating or printing steps. Thus, when the client receives a new card, it may have already consumed a certain number of personal identification numbers. If this number is higher than that tolerated by the system that manages the application, the client then receives a defective card which does not work. This situation is evidently detrimental for the service supplier and causes difficulties for clients.